A liquid crystal display (LCD) includes a liquid crystal alignment film. The liquid crystal alignment film is primarily made of polymer materials. The liquid crystal alignment film directs the alignment of liquid crystal molecules. When the liquid crystal molecules are moved by the influence of an electric field to display an image, the liquid crystal alignment film allows the liquid crystal molecules to be oriented in a predetermined direction. Generally, it is necessary to uniformly align the liquid crystal molecules in order to provide uniform luminance and a high contrast ratio to the liquid crystal device.
The conventional method of aligning the liquid crystal molecules includes coating a polymer film such as a polyimide on a substrate made of a material such as glass, and rubbing the surface of the substrate with a fiber such as nylon or polyester in a certain direction. However, the rubbing method may cause serious problems when fabricating a liquid crystal panel due to fine dust or electrostatic discharge (ESD) that may be generated while rubbing the polymer film with the fiber.
Significant research has focused on overcoming the problems associated with the rubbing method and to improve the viewing angle. For this purpose, vertical liquid crystal alignment has been developed. However, vertical alignment also has problems in that manufacturing processes are needed and luminance is reduced since a photolithography process is required on a color filter glass in order to obtain a wide viewing angle.
In order to solve the problems of the rubbing method, a photo-radiation method has recently been proposed to induce anisotropy to the polymer film by irradiating light on the film so as to align the liquid crystal molecules.
Polymers having photo-functional groups such as azobenzene, coumarin, chalcone, and cinnamate have been suggested as polymer film materials for photo-alignment methods. Such polymers are anisotropically photo-isomerized or photo-cross-linked by irradiation with polarized light, so as to provide anisotropy to the surface so that the polymer film can induce alignment of the liquid crystal molecules in a certain direction.
The material for the liquid crystal alignment film should have optical stability and thermal stability, as well as no after-image in order for it to be useful in a large liquid crystal display device (LCD). However, conventional photo-alignment materials have many troubles in this respect.
Further, conventional materials for the liquid crystal photo-alignment film are primarily polymeric with a main polymer chain and a side group with a photo-functional group that is capable of inducing photo-anisotropy, such as azobenzene or cinnamate. When such a material is used for a polymeric liquid crystal photo-alignment film, however, problems may result. For example, a significant amount of photo energy is required to induce the anisotropy. Also, a plurality of unreacted photo-functional groups can remain, which can negatively affect thermal stability, optical stability, and electro-optical characteristics.
In general, a liquid crystal display is fabricated by coating a liquid crystal photo-alignment agent on a glass substrate deposited with a transparent indium tin oxide (ITO) conductive layer and heating it to form a liquid crystal alignment film. Two substrates oppositely facing each other are then combined, and the liquid crystals implanted therebetween. Alternatively, a liquid crystal display can be fabricated by dripping liquid crystals on one substrate and combining the substrate with another oppositely facing substrate. Later generations of liquid crystal displays (5th generation or later) of medium- and large-sized product lines are typically manufactured using the latter method.
In general, a liquid crystal photo-alignment film is formed by coating a liquid crystal photo-alignment agent prepared by dissolving polyamic acid or polyimide in an organic solvent on a substrate in a flexo printing method, and then predrying and firing it. When the liquid crystal photo-alignment agent has bad printability, it may have a film thickness deviation, and thereby may have a negative influence on display characteristics of a liquid crystal display including the film.
In order to solve this problem, Japanese Patent Laid-Open Publication Pyong 8-208983 discloses a liquid crystal photo-alignment agent prepared by dissolving diethylene glycol diethylether in a solvent with excellent dissolvability against polyamic acid or polyimide. In addition, Korean Patent Laid-Open Publication No. 2005-0106423 discloses a liquid crystal photo-alignment agent with excellent printability, which is prepared by using diethylene glycol diethylether and dipropylene glycol monomethylether as a solvent.
Accordingly, a liquid crystal photo-alignment agent prepared by using the solvents has improved printability since it is rapidly spread out on a substrate. These methods, however, can cause aggregations at the ends of the substrate after printing, and thereby, the methods can fail to form a uniform film.